Window Alert 4 Hummingbird Decals Protect Wild Birds. Window Alert is a static-cling or low-tack decal that may be applied to home and office windows. The decals contain a component that brilliantly reflects ultraviolet sunlight. This ultraviolet light is invisible to humans but glows like a stoplight for birds. Birds have vision that is up to 12 times better than that of humans. Window Alert decals help birds see windows and avoid striking the glass. Window Alert decals help prevent wild birds from accidentally striking windows. Each decal has a special coating that reflects ultraviolet sunlight. The decals appear as frosted or etched glass. The decals measure approximately 4 across and are designed to be placed on the outside of a window. As a rough guide we recommend one decal for every 3 x 3 section of glass. Contents: 4 decals per envelope.